1. Field of the Invention
This invention relates generally to processes and electronic oscillating circuits, and, more particularly, to processes and oscillating circuits able to correct frequency variations in oscillating circuits caused by changes in temperature and other environmental conditions, and able to also correct frequency variations caused by environmental factors in non-crystal periodic sources.
2. Description of the Related Art
Oscillating circuits play a central and increasingly important role in digital and analog electronic systems. Digital devices require precise system timing, a function provided by oscillators and similar timing sources. Telecommunication and data transmission systems, which have analog and digital components, likewise rely on oscillators for modulation, demodulation, system clocking, and other functions.
A standard choice for a highly stable frequency source in such applications is a crystal-based oscillator or resonator. (Atomic frequency standards, while highly accurate, are undesirable in most such applications because of cost and packaging considerations.) While stable in comparison with non-crystal based resonating circuits, crystal oscillators and resonators nevertheless exhibit a degree of frequency instability owing to a crystal's inherent frequency response to temperature changes and to other environmentally influenced factors such as aging. See the paper titled Frequency-Temperature-Angle Characteristics of AT-Type Resonaters Made of Natural and Synthetic Quartz, Rudolf Bechmann, Proceedings of the IRE, November, 1956, pp. 1600-1607.
Current practice to correct such frequency instabilities follows two basic approaches. The first is represented by temperature compensated crystal oscillators (TCXOs) and digitally compensated crystal oscillators (DCXOs). In these designs, circuit elements sense the ambient temperature (which reflects the temperature of the crystal element) and the reactance loop of the oscillator circuit is adjusted to correct the frequency output. A DCXO differs from a TCXO primarily in the method employed to acquire temperature-related information used to compensate the oscillator circuit. A DCXO typically uses a temperature sensor, microprocessor and EPROM to acquire and store compensation data; a TCXO's compensation network uses analog devices solely, such as thermistors.
The second approach achieves frequency control by simply maintaining the oscillator's crystal element at a constant ambient temperature during operation, thus eliminating temperature as a cause of frequency variation. This approach is taken by the oven compensated crystal oscillator (OCXO). The relative success of these approaches varies. An OCXO can be manufactured which is significantly more accurate than a TCXO or DCXO. TCXO and DCXO oscillators are typically offered in the marketplace with accuracies from 5 ppm to 0.5 ppm. OCXO oscillators can be manufactured with accuracies from 0.5 ppm to 0.005 ppm. There is some overlap in accuracy between low end OCXOs and high end TCXOs and DCXOs. There are disadvantages with the OCXO relative to the TCXO and the DCXO, namely that it requires more power to operate, generates much waste heat, requires a substantial warn-up time, and occupies a bulky package. TCXO and DCXO oscillators have their own limitations, including relatively complex compensation networks (e.g., number of thermistors and other circuit elements to adjust the reactance loop) as well as the need to begin with a well-tuned, precise oscillator circuit and crystal element. These requirements make fabrication of TCXO and DCXO devices relatively elaborate and costly, although manufacture is generally less costly for TCXO and DCXO devices than for OCXOs.
Recent exemplars of contemporary practice include Watanabe et al. (U.S. Pat. No. 5,548,252, Digital Temperature Compensated Crystal Oscillator, Aug. 20, 1996). This oscillator uses a digital temperature compensated crystal oscillator (DTCXO) system with a memory that stores temperature compensation data received. Post et al. (U.S. Pat. No. 5,525,936, Temperature-Compensated Oscillated Circuit, Jun. 11, 1996), attempts to provide a temperature compensated oscillator circuit constructed with an oscillator controlled by a processor. The output frequency of the oscillator, or an external reference frequency, is used as a reference signal in conjunction with a dual mode oscillator that can be switched to provide temperature-dependent fundamental and third harmonic frequencies.
Connell et al. (U.S. Pat. No. 5,481,229, Low Power Temperature Compensated Crystal Oscillator, Jan. 2, 1996), shows a temperature compensated crystal oscillator constructed with a crystal oscillator circuit, a voltage controlled reactance element, a temperature compensation network, and a programmable DC-DC converter network having an output connected to the voltage controlled reactance element, or to the temperature compensation network, or both. Ishizaki et al. (U.S. Pat. No. 5,473,289, Temperature Compensated Crystal Oscillator, Dec. 5, 1995) has a temperature compensated crystal oscillator with an oscillation circuit, a temperature detecting circuit, and a control signal generating circuit, which is used as a reference frequency oscillator in a mobile communication device, such as a car telephone, a portable telephone, and a cordless telephone, a satellite communication device, and the like. Pucci et al. (U.S. Pat. No. 5,459,436, Temperature Compensated Crystal Oscillator With Disable, Oct. 17, 1995) discusses a temperature compensated crystal oscillator (TCXO) with a disable feature adapted to disable or enable temperature compensation The TCXO includes a crystal oscillator and a temperature compensation circuit.
Our study of contemporary practice leads us to conclude that contemporary practice fails to provide an oscillating circuit capable of effectively generating a periodic signal exhibiting a stable period in the presence of frequency fluctuations caused in the circuit by the effect of temperature changes and other changing environmental conditions such as crystal aging on the crystal element.